Fix typo in probabilistic inference task definition

bayesian.tex

@@ -32,7 +32,7 @@
   $\mathbf{X}$ is a set of ``query variables'' and $\mathbf{E}$ is a
   set of ``evidence variables'' assigned by an
   \hyperref[def:event]{event} $\mathbf{e}$. The remaining variables
-  $\mathbf{Y}$ are refereed to as ``hidden''.
+  $\mathbf{Y}$ are referred to as ``hidden''.
 
   This problem can by solved using ``Inference by Enumeration'':
   \begin{enumerate}